This invention relates to a system for gas concentration control, and more particularly to an apparatus such as a blood oxygenator which allows for the independent control of oxygen (O.sub.2) and carbon dioxide (CO.sub.2) transfer with regard to the blood without increasing the amount of O.sub.2 and CO.sub.2 dissolved in the patients blood can be more precisely controlled.
In various types of surgical procedures it is often necessary to perform a treatment whereby the patients blood is subject to a bypass flow outside of the patients body, and an apparatus such as an oxygenator is employed. Such oxygenators are used in open-heart surgery and other operations and treatments of the body when it is necessary to establish an extracorporeal circulation system for temporarily assuming the functions of the heart and lungs of the patient. In such a system the oxygenator operates to perform the function usually performed by the lungs of the patient, i.e., the life-supporting transfer of oxygen into the blood and carbon dioxide out of the blood. The oxygenator is used in association with a pump which performs the function of the heart to cause circulation of the blood. Thus, early versions of the oxygenator were often referred to as "heart-lung" machines. The early heart-lung machines were typically rotating discs which passed through a pool of blood, but were only partially immersed therein such that the free surface of the disc exposed the blood to oxygen accomplished some gas transfer. After this, bag-type oxygenators were introduced which were superior to the disc oxygenators, but which left much to be desired.
At the present time two principle types of blood oxygenators are used which have proven efficient, provide minimal blood trauma, are convenient to set up and operate, are cost effective and have provided excellent clinical results, i.e. bubble oxygenators and membrane oxygenators.
In a membrane oxygenator, a thin, highly gas permeable membrane is placed between the gas and blood. Venous blood flows along one side of the membrane and gas is on the other side. A concentration gradient is established so that when the partial pressure for oxygen is higher in the ventilating gas than the partial pressure for oxygen in the venous blood, oxygen will diffuse across the membrane into the blood. Bubble oxygenators simply diffuse gas bubbles into venous blood. The oxygenated blood is typically defoamed before it is ready for delivery to the patient.
The typical bubble oxygenator is constructed of three chambers that are connected in series with each other, i.e. (1) a gas exchange or bubble chamber in which gas is dispersed as bubbles into the venous blood through small holes in a distributing manifold or sparger that is particularly used to create bubbles of the proper diameter and to disperse them effectively in the venous blood i.e., create foam and bubbles, and an effective mixture of gas and blood such that transfer o the oxygen into the blood takes place; (2) a defoaming or debubbling chamber wherein after gas transfer is completed, coalescence of the foam and the removal of the remaining bubbles is performed., and (3) a settling chamber in which the defoamed and oxygenated blood settles prior to being pumped back to the patient. Typically a heat exchange element is used in the bubble chamber for maintaining the blood temperature as for hypothermia.
Various prior art examples of blood oxygenators and gas-liquid type of transfer apparatus known in the art are described in U.S. Pat. Nos. 3,065,748 (illustrates two outlets 16 and 17), 3,256,883, 3,493,347 (two inlets 18 and 29), 4,073,622, 4,138,288, 4,182,739, 4,203,944, 4,203,945, 4,228,125, 4,231,988, 4,272,373 (separate inlets for gas and water), 4,336,224, 4,370,151, 4,374,O88, 4,396,584, 4,407,777, 4.440,722, 4,493,692 (two separate sources for O.sub.2 and O.sub.2 /CO.sub.2) and 4,533,516.
In all bubble type blood oxygenators two types of gas transfer must take place. One is oxygen O.sub.2) which is transferred into the blood and the other is carbon dioxide (CO.sub.2) which is transferred out of the blood. Typically a bubble oxygenator provides a single gas inlet means for directing an oxygen bearing gas to a sparger which then disburses finely divided air bubbles into the blood. These bubble oxygenators also include an outlet means for carrying a gas bearing both oxygen and carbon dioxide out from the oxygenator.
In such presently available bubble oxygenators the one gas inlet only allows for the regulation of the gas flow to the sparger, and as a result for the control of the amount of the oxygen bearing gas microbubbles being delivered to the blood. This is generally performed by an adjustable gas valve positioned in the gas line. The amount of gas flowing to the blood directly affects the amount of oxygen delivery and carbon dioxide removal from the blood. In particular, microbubbles created by the sparger diffuse into the blood. The oxygen passes into the blood and the carbon dioxide passes into the bubbles through the bubble walls.
It is well recognized that the benefit of forming microbubbles from a given volume of gas is to maximize the total blood to gas interface area. That is, the rate of diffusion across the blood-gas interface, as defined by the bubble walls remains constant. By forming microbubbles the overall blood-gas interface area, as defined by the bubble walls is maximized for the given gas quantity.
The amount of oxygen delivered to the blood, as measured by the partial pressure of the oxygen (PO.sub.2), as well as the amount of carbon dioxide being removed from the blood is increased by increasing the gas flow rate to the oxygenator. By decreasing the flow rate of the gas the amount of the oxygen delivery and carbon dioxide removal is reduced. However, during, for example, open heart surgery when the patient is being cooled or is cooled down, it does not take as much oxygen to oxygenate the blood of the patient. While under such conditions it is desirable to reduce the oxygen exchange rate, the rate of carbon dioxide removal from the blood remains the same or even increases during such operations. The disadvantage with presently available oxygenators is that such oxygenators do not possess the able to remove or blow off CO.sub.2 without also having to increase the rate of oxygen exchange. That is, in order to blow off more CO.sub.2, it is necessary to provide a higher gas flow rate which increases the amount of O.sub.2 being delivered to the blood. Higher partial pressures of the O.sub. 2 are not wanted because of various undesirable side effects. It would therefore be desirable in a bubble oxygenator to be able to maintain the CO.sub.2 removal from the blood while at the same time allowing for a lower O.sub.2 transfer into the blood.